Method for reducing crack rate of 45 steel quenched and tempered flange plate

Abstract

The invention discloses a method for reducing the crack rate of a 45 steel quenched and tempered flange plate. The method is suitable for quenching and tempering of small and medium-sized mechanical parts with large structure thickness differences. According to the method, a water-oil double-medium quenching process is adopted, ordered transformation of a martensite structure is achieved by accurately controlling the initial rapid cooling time, the quenching internal stress is remarkably reduced, and quenching cracking is prevented. Experimental results show that compared with a traditional single brine quenching process, the method can reduce the crack rate of the flange plate from 50% or above to 5% or below, and the quenching and tempering qualification rate is effectively increased. The method is high in process controllability and suitable for batch production, and has good popularization and application prospects.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of heat treatment process, and particularly relates to a method for reducing the crack rate of a 45 steel quenched and tempered flange. BACKGROUND

[0002] 45 steel is a commonly used medium carbon structural steel, and is widely used in flange, gear, shaft and other mechanical parts which need to be quenched and tempered to obtain strong and tough matching performance. However, during the quenching process of 45 steel, especially for the forgings with complex shape or obvious thickness difference, cracks are prone to occur during the quenching process. The traditional quenching process usually adopts salt water quenching to improve the hardness and hardenability, but under the condition of strong cooling, especially in the thin area, asynchronous microstructure transformation is prone to occur, which leads to severe volume expansion stress and causes quenching cracks. Especially in the flange forgings with large structural thickness difference, the crack scrap rate is often more than 50%, which seriously affects the production efficiency and cost. Therefore, there is an urgent need for a heat treatment method which can control the microstructure stress and effectively reduce the quenching crack risk. SUMMARY

[0003] The purpose of the application is to provide a method for reducing the crack rate of a 45 steel quenched and tempered flange, which adopts double medium quenching technology to realize the purpose of reducing the crack rate and improving the qualified rate through fast-slow segmented cooling.

[0004] To solve the above technical problems, the application adopts the following technical scheme:

[0005] The application provides a method for reducing the crack rate of a 45 steel quenched and tempered flange, which comprises the following steps:

[0006] Quenching heating:

[0007] Heating to 820-825 DEG C, and keeping for 60-85 minutes;

[0008] Double medium quenching:

[0009] The heated workpiece is put into salt water for rapid cooling for 3-5 seconds, and then is quickly transferred to quenching oil for slow cooling to room temperature; wherein the cooling speed in the range of 820-600 DEG C is 180-220 DEG C / s, and the cooling speed in the range of 600-350 DEG C is 120-150 DEG C / s;

[0010] Tempering treatment:

[0011] The tempering is carried out within 3-5 hours after the quenching is completed, the tempering temperature is 520-530 DEG C, the holding time is 2-3 hours, and air cooling is adopted.

[0012] Further, it further comprises the preparation before quenching and tempering: the flange parts after forging are subjected to conventional descaling treatment, and the surface is kept clean and dry; the furnace temperature is controlled to ensure uniform heating and prevent thermal cracking.

[0013] Further, the salt water is 10% NaCl salt water.

[0014] Further, the salt water temperature is 20-40 DEG C.

[0015] Further, the method is suitable for 45 steel meeting S78002-2016 standard.

[0016] Further, the 45 steel mass percentage chemical composition is: C: 0.42-0.50%, Si: 0.17-0.37%, Mn: 0.50-0.80%, P≤0.025%, S: 0.015-0.030%, Cr≤0.20%, Ni≤0.30%, the rest is Fe and impurities.

[0017] Further, the 45 steel mass percentage chemical composition is: C: 0.49%, Mn: 0.67%, Si: 0.22%, Cr: 0.09%, Ni: 0.03%, P 0.015%, S 0.015%, the rest is Fe and inevitable impurities.

[0018] Further, after tempering, tempered sorbite structure is obtained, and the hardness is controlled between 25HRC and 30HRC.

[0019] Further, it is suitable for flange plate type structural parts with wall thickness between 8mm and 12mm and thickness difference greater than 3 times.

[0020] Further, the quenching process can automatically time switch medium, reduce operation fluctuation, and is suitable for batch production line implementation.

[0021] Compared with the prior art, the beneficial technical effects of the present application are:

[0022] The appearance crack of the quenched and tempered part is checked, the metallographic structure is observed, and the mechanical property is detected; no quenching crack, uniform structure, and the hardness and dimensional stability meet the product standard. After tempering, tempered sorbite structure is obtained, and the hardness is controlled between 25HRC and 30HRC. The quenched and tempered sorbite grain size is 10-15 mu m, and the crack distribution is concentrated in the original thin-wall stress area.

[0023] Compared with the traditional single salt water quenching, the method has the following advantages: (1) the crack rate is significantly reduced: the crack scrap rate of more than 50% of the original process is reduced to less than 5%; (2) the internal stress and deformation are reduced: the double medium cooling path relieves the superposition effect of high temperature and structure stress; (3) the hardenability and structure stability are considered: under the premise of meeting the hardness requirement, the risk of burst caused by martensite mutation is avoided; (4) simple operation and suitable for batch production: no equipment updating is needed, and only the cooling medium and operation time sequence optimization can be realized; (5) wide applicability: it can be popularized to other medium carbon steel or similar structure parts in the quenching and tempering scene. DETAILED DESCRIPTION

[0024] The method for reducing the crack rate of the 45 steel quenched and tempered flange plate of the application will be further described below.

[0025] Embodiment 1: This embodiment is one of the preferred embodiments of the various embodiments of the application.

[0026] The material is 45 steel conforming to S78002-2016 standard;

[0027] Heat treatment process: quenching heating temperature: 820℃, holding time 60 minutes; first salt water (20-30℃) cooling for 4 seconds, then oil cooling (40-50℃) to room temperature; tempering temperature: 525℃, holding time 2.5 hours, air cooling;

[0028] Workpiece batch: 200 pieces;

[0029] Results: only 7 pieces of cracked parts are found, the crack rate is 3.5%, which is significantly reduced; the quenching and tempering qualified rate is 96.5%.

[0030] Embodiment 2:

[0031] The quenching holding time is extended to 70 minutes, and the salt water time is 3 seconds;

[0032] Results: the crack rate is further reduced to 2%, and there is no obvious quenching distortion in the appearance of the parts; structure: tempered sorbite, uniform grain, good performance.

[0033] Embodiment 3:

[0034] The oil temperature is set to 60℃, and the rest of the parameters are the same as those in embodiment 1;

[0035] Results: the crack rate is 4%, which is slightly higher than that in embodiment 2, but still significantly better than that in the comparative example.

[0036] Comparative example 1:

[0037] Quenching process: after 825℃×85 minutes holding, 10% NaCl salt water cooling to room temperature; tempering: 530℃×2.5 hours;

[0038] Batch: 500 pieces;

[0039] Cracks: 274 pieces, crack rate 54.8%, severely over-standard;

[0040] Crack characteristics: concentrated in the thin-walled end face, typical quenching cracks.

[0041] Comparative Example 2:

[0042] Single oil cooling medium (50℃) is used, and the cooling speed is insufficient;

[0043] Part of the thick section is not quenched, and the hardness is not up to standard;

[0044] No cracks, but the qualified rate is less than 80%, and the structure is uneven.

[0045] Comparative Example 3:

[0046] After quenching, it is not immediately transferred to oil cooling, but only naturally cooled to room temperature;

[0047] Severe distortion and cracks occur, and the crack rate is close to 50%.

[0048] Comparison of crack rate and performance under each process:

[0049]

[0050]

[0051] From the comparison of the examples and comparative examples, it can be obviously seen that: (1) the quenching crack rate is significantly reduced: after using the double-medium quenching process of the present application, the crack rate is greatly reduced from 54.8% in the traditional single salt water quenching to less than 5%, effectively solving the cracking problem of thick-thin uneven structure parts in the quenching process; (2) the microstructure is more uniform and fine: the temper sorbite structure formed by the present application has fine and uniform grain distribution, which significantly improves the stability of the microstructure and the comprehensive mechanical properties compared with the coarse and uneven microstructure in the comparative examples; (3) the hardness and quenching qualified rate are both improved: under the premise of ensuring the hardness up to standard (25-30HRC), the quenching qualified rate can reach more than 96%, while in the comparative examples, the qualified rate is generally less than 80% due to cracking or insufficient quenching; (4) the cooling process is more controllable and adaptable: the double-medium cooling path effectively balances the contradiction between rapid cooling and stress release, avoiding the problems of cracks or insufficient hardness caused by excessive or insufficient cooling speed; (5) it has good engineering application prospect: the process parameters of the present application are clear, and the operation mode is mature, which is suitable for mass production of heat treatment production line, and is expected to be widely used in 45 steel flange, shaft, gear and other complex structure quenching and tempering parts heat treatment scenes.

[0052] The above described embodiments are only to illustrate the preferred modes of the present application, and are not intended to limit the scope of the present application. Any modification and improvement made by those skilled in the art to the technical solutions of the present application without departing from the design spirit of the present application shall fall within the protection scope of the present application as defined by the claims.

Claims

1. A method for reducing the crack rate of a 45 steel quenched and tempered flange, characterized in that, Comprising: quenching heating: heating to 820-825 DEG C, holding for 60-85 minutes; double medium quenching: after heating, the workpiece is put into salt water, and is rapidly cooled for 3-5 seconds; then is rapidly transferred to quenching oil to slowly cool to room temperature; tempering treatment: tempering is carried out within 3-5 hours after quenching is completed; the tempering temperature is 520-530 DEG C, the holding time is 2-3 hours, and air cooling is adopted.

2. The method for reducing the crack rate of 45 steel quenched and tempered flanges according to claim 1, characterized in that, It also comprises preparation before quenching and tempering: the flange disc parts after forging are subjected to conventional descaling treatment, and the surface is kept clean and dry; the furnace temperature is controlled to ensure uniform heating and prevent thermal crack.

3. The method of claim 1, wherein the method is characterized by, The salt water is 10% NaCl salt water.

4. The method for reducing the crack rate of 45 steel quenched and tempered flanges according to claim 1 or 3, characterized in that, The salt water temperature is 20-40 DEG C.

5. The method of reducing the crack rate of 45 steel quenched and tempered flanges according to claim 1, characterized in that, The method is suitable for 45 steel meeting the S78002-2016 standard.

6. The method for reducing the crack rate of 45 steel quenched and tempered flanges according to claim 5, characterized in that, The chemical composition of the 45 steel in percentage by mass is: C: 0.42-0.50%, Si: 0.17-0.37%, Mn: 0.50-0.80%, P≤0.025%, S: 0.015-0.030%, Cr≤0.20%, Ni≤0.30%, and the rest is Fe and impurities.

7. The method for reducing the crack rate of 45 steel quenched and tempered flanges according to claim 6, characterized in that, The chemical composition of the 45 steel in percentage by mass is: C: 0.49%, Mn: 0.67%, Si: 0.22%, Cr: 0.09%, Ni: 0.03%, P 0.015%, S 0.015%, and the rest is Fe and unavoidable impurities.

8. The method of reducing the crack rate of 45 steel quenched and tempered flanges according to claim 1, characterized in that, After tempering, tempered sorbite structure is obtained, and the hardness is controlled between 25HRC and 30HRC.

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